This invention relates to a spread-spectrum signal receiver for receiving a signal (referred to as a spread-spectrum signal) that has been spread by a direct-sequence (DS) scheme. More particularly, the invention relates to a spread-spectrum signal receiver for receiving a spread-spectrum signal obtained by rendering transmit data and control data, which have been modulated at different powers, into signals that do not influence each other, e.g., signals that are mutually orthogonal, multiplexing these signals and transmitting the multiplexed signal from a transmit side.
In spread-spectrum communications, W-CDMA (Wideband Code Division Multiple Access), which employs direct-sequence spreading, is one of the third-generation mobile communications systems the standardization of which is being forwarded by the 3GPP (3rd Generation Partnership Project).
With CDMA, as shown in FIG. 9, a mobile station, which is a spread-spectrum signal transceiver, has a first modulator 1a for applying BPSK modulation (see FIG. 10) to control data that includes pilot data, and a first spreader 1b for applying spread-spectrum modulation using a spreading code for the control data. The mobile station further includes an encoding circuit 1c for subjecting the transmit data to suitable encoding such as convolutional coding a second modulator 1d for subsequently applying BSPK modulation and a second spreader 1e for spreading the resultant signal using a spreading code for the transmit data. The mobile station further includes a multiplexer 1f for mapping the control data and transmit data, which have been spreaded by the first and second spreaders, as an I-axis component (I-channel component) and Q-axis component (Q-channel component) of an I-Q complex plane, as illustrated on the right side of FIG. 10, and multiplexing the resulting signals, and a radio transmitter unit 1g for subjecting the multiplexed signal to frequency conversion and high-frequency amplification and transmitting the resulting signal from an antenna 1h. It should be noted that the I and Q channels are referred to also as data and control channels, respectively.
An uplink signal from the mobile station to a base station has a frame format shown in FIG. 11. One frame has a duration of 10 ms and is composed of 15 slots S0 to S14. Transmit data is mapped to the I channel (data channel) and control data, which is data other than the transmit data, is mapped to the Q channel (control channel). Each of the slots S0 to S14 of the data channel that transmits the transmit data is composed of n bits, where n varies depending upon symbol rate. The symbol rate will be 7.5 (=5×15/10×10−3) kbps if n=5 holds; 15 (=10×15/10×10−3) kbps if n=10 holds; 30 kbps if n=20 holds, and so forth.
Each slot of the control channel that transmits the control data is composed of 10 bits, and the symbol rate is a constant 15 kbps. Each slot transmits a pilot, transmission-power control data TPC, a transport format combination indicator TFCI and feedback information FBI. The pilot is utilized on the receive side for coherent detection and SIR (Signal to Interference Ratio) measurement, the TPC is utilized for control of transmission power, the TFCI transmits the symbol rate of the data and the number of bits per frame, etc., and the FBI is for controlling transmission diversity at the base station.
Thus, there are instances where the symbol rates on the data and control channels differ. In such case the spreading factor [=(symbol rate)/(chip rate)] on the data channel differs from that on the control channel. For example, (1) if the symbol rate of the data channel is lower than that (15 kbps) of the control channel, then the spreading factor of the data channel will be larger than that of the control channel, and (2) if the symbol rate of the data channel is higher than that (15 kbps) of the control channel, then the spreading factor of the data channel will be smaller than that of the control channel. The larger the spreading factor, the higher the process gain. Accordingly, in a W-CDMA system, transmission power for which the spreading factor is larger is reduced to lower the total transmission power. In other words, with W-CDMA, the control and data channels are subjected to BPSK and spread-spectrum modulation at powers that differ from each other, the spread-spectrum modulated signals are mapped on an I-Q complex plane and multiplexed and the multiplexed signal is transmitted.
If, by way of example, the spreading factor of the data channel is larger than that of the control channel, then, as shown in FIG. 12, the apparatus of FIG. 9 is further provided with multipliers 1h, 1i, the multiplier 1h multiplies the BPSK modulation output of the second modulator 1d of the data channel by β (β<1) and the multiplier 1i multiplies the BPSK modulation output of the first modulator 1a of the control channel by 1 (i.e., leaves this output unchanged). The first and second spreaders 1b, 1e thenceforth spread-spectrum modulate the outputs of the multipliers 1i, 1h, respectively, the multiplexer 1f maps the spread-spectrum modulated signals of the respective channels on the I-Q complex plane, as illustrated in FIG. 13A, and multiplexes the resultant signals, and the radio transmitter unit 1g subjects the multiplexed signal to a frequency conversion and high-frequency amplification and transmits the resulting signal from the antenna 1h. By thus lowering the transmission power of the channel having the larger spreading factor, the total transmission power can be controlled (reduced).
Further, if the spreading factor of the data channel is made smaller than that of the control channel, the multiplier 1h multiplies the BPSK modulation output of the second modulator 1d by 1 and the multiplier 1i multiplies the BPSK modulation output of the first modulator 1a by β (β<1). The multiplexer 1f maps the spread-spectrum modulated signals of the respective channels on the I-Q complex plane, as illustrated in FIG. 13B, and multiplexes the resulting signals. As a result, the total transmission power can be reduced by lowering the transmission power of the channel having the larger spreading factor.
FIG. 14 is a block diagram illustrating one channel of the receiver section of a base station, which is a spread-spectrum signal transceiver. The base station has a radio unit 2a for frequency-converting a high-frequency signal received from an antenna ATN to a baseband signal; a quadrature demodulator 2b for subjecting the baseband signal to quadrature detection, converting the analog in-phase component (I component) and analog quadrature component (Q component) to digital data and inputting the data to a searcher 2c and fingers 2d1˜2dn. Upon receiving input of a direct-sequence signal (DS signal) that has been influenced by the multipath effect, the searcher 2C detects multipath interference by performing an autocorrelation operation using a matched filter and inputs despreading-start timing data and delay-time adjustment data of each path to the fingers 2d1˜2dn. A control-channel despreader 3a of each of the fingers 2d1˜2dn subjects a direct wave or delayed wave that arrives via a prescribed path to despread processing using a code identical with the spreading code for the control channel, integrates the results of despreading, then applies delay processing that conforms to the path and outputs a control-data signal. A data-channel despreader 3b subjects a direct wave or delayed wave that arrives via a prescribed path to despread processing using a code identical with the spreading code for the data channel, integrates the results of despreading, then applies delay processing that conforms to the path and outputs a transmit-data signal.
A channel estimation unit 3c estimates the phasing characteristic of the propagation path using the pilot signal contained in the despread control-data signal, executes channel estimation which compensates for the effects of phasing, and outputs a channel estimation signal. Channel compensation units 3d, 3e multiply the despread control-data signal and despread transmit-data signal by the complex-conjugate signal of the channel estimation signal to thereby compensate for phasing. A RAKE combiner 2e combines the control-data signals output from the fingers 2d1˜2dn and outputs the result as a soft-decision data sequence. A decision unit 2f renders the soft-decision data into hard decision data, and a decoder 2g applies error-correction decoding processing to the hard-decision data, which is output from the decision unit 2f, decodes the control data that prevailed prior to encoding and outputs the decoded data. It should be noted that the soft-decision data, rather than being made hard-decision data by the decision unit 2f, can be subjected to soft-decision error-correction processing by the decoder 2g as is, whereby the control data is decoded and output. A power-ratio calculation unit 2h calculates the ratio (Pd/Pc=β2) of data-channel signal power Pd to control-channel signal power Pc from the rate information (symbol rate of the data) included in the TFCI bit of the control data and outputs β.
A RAKE combiner 2i combines the control-data signals output from the fingers 2d1˜2dn and outputs the result as a soft-decision data sequence. A delay add-on unit 2j performs a time adjustment by delaying the soft-decision data sequence long enough for the calculation of the power ratio. A signal level changing unit 2k multiplies the soft-decision data level of the data channel by 1/β (the level of the transmit-data signal is multiplied by β on the transmit side) and then renders the data into hard decision data. A decoder 2m applies error-correction decoding processing to the hard-decision data, which is output from the signal level changing unit 2k, decodes the transmit data that prevailed prior to encoding and outputs the decoded data. It should be noted that the soft-decision data that has been multiplied by 1/β, rather than being made hard-decision data by the signal level changing unit 2k, can be subjected to soft-decision error-correction processing by the decoder 2m as is, whereby the transmit data is decoded and output.
Thus, in a case where a transmit-data signal and control-data signal are transmitted at different powers with the conventional spread-spectrum signal receiver, power-ratio information is computed based upon symbol-rate information contained in the control data, the level of the transmit-data signal is changed based upon this information and then error-correction decoding processing is applied. With this method, however, a lengthy processing delay occurs by the time the power-ratio information is acquired. For example, with W-DCMA, the symbol-rate information is transmitted over one frame (10 ms), as mentioned earlier, and therefore a minimum of 10 ms is required to acquire the power-ratio information. Further, the symbol-rate information is encoded in order to improve its reliability, i.e., its reception characteristic, and therefore additional processing time is necessary to decode this information.
Information demanding delivery in real time, as in the case of voice communication, requires a short communication delay, i.e., a short processing time. Further, a short processing time is desirable also in interference suppression techniques, such as in an interference canceller. Also required is a circuit such as a memory for adding a processing delay onto the data channel. The result is an increase in the scale of the circuitry that is proportional to the amount of processing delay.